The present invention relates generally to a power meter with automatic configuration capability.
Electrical power is provided to many devices, such as large motors, by three separate cables, each of which supplies a single phase of three phase-power. In an ideal system, each of the phases within the respective cable has a phase angle which is generally 120 degrees apart from the other phases. Accordingly, the total power flowing through the three cables to a three-phase load (or from a three-phase generator) may be computed based upon the phase adjusted voltages and the phase adjusted currents.
A sub-metering system generally includes a separate transformer installed on each respective cable of the three cables of a three-phase system within a customer's power box containing the electrical panel. Each transformer senses a changing current within a respective cable and produces an output voltage or current proportional to the changing current. A measuring circuit is electrically connected to the three transformers and receives each of the transformer output voltages or currents. The measuring circuit is also electrically connected to the three cables by voltage “taps” to measure the voltage therein. The voltage “tap” measurements are preferably obtained by an electrical connection to the interface between each phase of the respective cable and the panel. The measuring circuit calculates the power usage of the respective three phases using the output voltages from the transformers and the voltages sensed by the three voltage “taps.”
Flexibility has favored adoption of digital current and power meters, known as branch circuit monitors, incorporating data processing systems typically comprising one or more microprocessors or digital signal processors (DSP) that periodically read the output of each of the voltage and current transducers, calculate the current or voltage at the respective transducer and display or store the results. In addition, the data processing unit periodically may calculate the power and other electrical parameters, such as active power, apparent power and reactive power that quantify electricity distribution and consumption. The calculated parameters are typically output to a display for immediate viewing and/or transmitted from a communications interface to another data processing system, such as a building management computer for remote display or further processing, for example formulating instructions to automated building equipment.
Branch circuit monitors are commonly incorporated in networks that utilize the MODBUS® RTU serial communication protocol. The MODBUS protocol, first published in 1979, enables serial communication between up to 247 interconnected network devices and has become a de facto standard communications protocol for connecting industrial electronic devices. Each device is given a unique address and each query contains the address of the intended device. A device will not respond to a query unless it contains that device's address. A basic MODBUS query can control or read an input/output port of a receiving device or instruct the receiving device to change or send back one or more values contained in its memory.
While the MODBUS protocol is a de facto standard for industrial electronic devices, it does have its limitations. For example, since MODBUS was developed in the 1970s to communicate with programmable logic controllers (PLCs), the number of supported data types is limited to those utilized by PLCs at that time and does not include large binary objects. Also, the number of addressable registers in a MODBUS device is limited which can affect the device's performance and utility. For example, a branch circuit monitor monitoring a large number of circuits and determining several electrical measurement parameters for each circuit generates substantial amounts of data and the limited number of addressable registers can limit the number of circuits monitored by the branch circuit monitor and/or the types of data that can be produced and communicated by the monitor. Also, the configuration of such a device can be cumbersome and require significant computational resources to determine all the potential data that may be desired.
What is desired therefore is a power meter that is computationally efficient.
The foregoing and other objectives, features, and advantages of the invention may be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.